The steam generators of pressurized water nuclear reactors comprise a bundle consisting of a very large number of tubes the ends of which are crimped into a very thick tube plate.
During long-term use of such a steam generator, cracks causing a leak may appear on the sides of some tubes in the bundle. As the walls of the tubes from the separation between the primary fluid of the reactor and the water supplied to the steam generator, these leaks must be detected and repaired by servicing of the steam generator during a stoppage of the nuclear reactor.
Such repairs are well known and involve either plugging of the end of the tubes which leak, or lining of the cracked part of these tubes.
In the case of plugging, a hollow cylindrical component closed at one of its ends, the outer diameter of which is slightly less than the inner diameter of the tube in its crimped part, is introduced into the end of the tube crimped into the tube plate. Diametrical expansion of this cylindrical component is then carried out in a joining region close to the entry side of the tube plate, until the outer surface of the cylindrical component is in perfect contact with the inner surface of the tube. Attachment of the cylindrical component which forms the plug is completed by a crimping of its part which has undergone diametrical expansion within the tube and, if required, by welding its end, ensuring perfectly tight sealing of the tube.
In the case of lining, a cylindrical component or sleeve, of an outer diameter less than the inner diameter of the tube and open at its two ends, is introduced into the tube to be lined through the entry side of the tube plate. The length of the sleeve is generally greater than the thickness of the tube plate, and the sleeve is introduced into the tube plate until its end is flush with the end of the tube. The other end of the sleeve then projects from the outlet side of the tube plate. In the case of a lining process which was recently developed and which can preferably be applied to the peripheral tubes of the steam generator, a sleeve of a length substantially less than the thickness of the tube plate is employed. The sleeve is introduced into the tube so that it projects on one side from the outlet side of the tube plate. The other end of the sleeve is then situated within the part of the tube which is crimped into the tube plate and in a region substantially away from the entry side.
In both cases, the fastening of the sleeve in the tube is carried out by diametrical expansion followed by crimping of the sleeve in two regions near its ends. One of these regions is within the tube plate and the other region, beyond the outlet side of this plate, in an uncrimped part of the tube.
The crimping in this region situated outside to the tube plate is carried out so that the sleeve undergoes a plastic deformation and the tube an elastic deformation only. Therefore, the crimping is ensured by the radial stresses of elastic origin which operate in the tube.
Both in the case of plugging the tubes and in the case of lining, it is essential to carry out a thorough cleaning of the inner surface of the tube and the outer surface of the cylindrical component in the joining regions, in order to obtain a perfect tightness of these joints. The cleaning of the inner surface of the tube is an operation which is delicate and difficult to perform, especially in the case of a steam generator of a nuclear reactor, the tubes of which have a certain radioactivity and are accessible only with difficulty.
Moreover, despite thorough cleaning of this inner surface, the joint finally obtained is not always perfectly tight, and it is often necessary to complete the repairs with a welding of the end of the plug or of the sleeve on the tube.
Such a welding operation may lead to undesirable structural modifications or deformations of the tube.
Plugs or sleeves containing channels or grooves on their outer surface have also been proposed. However, such machinings have only a very limited effect on the tightness of the joint obtained by the crimping of the plug or of the sleeve, in the tube, the projecting parts of the sleeve the hardness of which is close to that of the tube, being crushed against the inner surface of the tube during the crimping.
It has also been proposed, for connecting tubes with a sieve surrounding the connection end parts of the tubes, to place a sealing material containing hard particles between the sleeve and the tubes, then to press the sleeve against the tube, so that the hard particles are embedded both in the sleeve and the tube. Such a process cannot be applied in the case of the lining or plugging of a steam generator tube, as it requires use of a sealing material like rubber or plastic in which the particles are embedded.
Finally, it has also been proposed to carry out the lining by welding or brazing the sleeve within the tube. Such an operation is extremely complex and prolongs significiantly the servicing time of the steam generator.